7.写个应用程序测试一下

  测试平台：ubuntu 16.04
  编译命令：gcc play_to_cap.c -o play_to_cap -lasound
  功能：边播放，边录音

play_to_cap.c

+#include <stdio.h>
+#include <fcntl.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+#include <alsa/asoundlib.h>
+
+
+#define PLAYBACK_FILE "R48K-16LE-CH1.pcm"
+#define CAPTURE_FINE "capture.pcm"
+
+#define PCM_NAME	"hw:CARD=mycodec,DEV=0"
+#define RATE 		48000
+#define FORMAT		SND_PCM_FORMAT_S16_LE
+#define CHANNELS	1
+
+snd_pcm_hw_params_t *hw_params;
+
+int print_all_pcm_name(void) {
+	char **hints;
+	/* Enumerate sound devices */
+	int err = snd_device_name_hint(-1, "pcm", (void***)&hints);
+	if(err != 0)
+	   return -1;
+
+	char** n = hints;
+	while(*n != NULL) {
+		char *name = snd_device_name_get_hint(*n, "NAME");
+
+		if(name != NULL && 0 != strcmp("null", name)) {
+			printf("pcm name : %s\n",name);
+			free(name);
+		}
+		n++;
+	}
+
+	snd_device_name_free_hint((void**)hints);
+	return 0;
+}
+
+snd_pcm_t *open_sound_dev(snd_pcm_stream_t type,const char *name, unsigned int rate, int format,int channels,snd_pcm_uframes_t *period_frames) {
+	int err;
+	snd_pcm_t *handle;
+	int dir = 0;
+	
+	printf("rate=%d, format=%d, channels=%d\n",rate,format,channels);
+
+	if((err = snd_pcm_open(&handle, name, type, 0)) < 0) {
+		return NULL;
+	}
+	   
+	if((err = snd_pcm_hw_params_malloc(&hw_params)) < 0) {
+		printf("cannot allocate hardware parameter structure (%s)\n",
+			 snd_strerror(err));
+		return NULL;
+	}
+			 
+	if((err = snd_pcm_hw_params_any(handle, hw_params)) < 0) {
+		printf("cannot initialize hardware parameter structure (%s)\n",
+			 snd_strerror(err));
+		return NULL;
+	}
+
+	if((err = snd_pcm_hw_params_set_access(handle, hw_params, SND_PCM_ACCESS_RW_INTERLEAVED)) < 0) {
+		printf("cannot set access type (%s)\n",
+			 snd_strerror(err));
+		return NULL;
+	}
+
+	if((err = snd_pcm_hw_params_set_format(handle, hw_params, format)) < 0) {
+		printf("cannot set sample format (%s)\n",
+			 snd_strerror(err));
+		return NULL;
+	}
+
+	if((err = snd_pcm_hw_params_set_rate_near(handle, hw_params, &rate, 0)) < 0) {
+		printf("cannot set sample rate (%s)\n",
+			 snd_strerror(err));
+		return NULL;
+	}
+
+	if((err = snd_pcm_hw_params_set_channels(handle, hw_params, channels)) < 0) {
+		printf("cannot set channel count (%s)\n",
+			 snd_strerror(err));
+		return NULL;
+	}
+
+	if((err = snd_pcm_hw_params(handle, hw_params)) < 0) {
+		printf("cannot set parameters (%s)\n",
+			 snd_strerror(err));
+		return NULL;
+	}
+	
+	if(period_frames != NULL) {
+		//获取一个周期有多少帧数据
+		if((err =snd_pcm_hw_params_get_period_size(hw_params, period_frames, &dir)) < 0){
+			printf("cannot get period size (%s)\n",
+				snd_strerror(err));
+			return NULL;
+		}
+	}
+	
+	snd_pcm_hw_params_free(hw_params);
+
+	return handle;
+}
+
+
+int main(void) {
+	int p,err;
+	snd_pcm_t *playback_handle;
+	snd_pcm_t *capture_handle;
+	int pfd,cfd;
+	snd_pcm_uframes_t period_frames;
+	int size2frames;
+	
+	printf("program running ...\n");
+	
+	//查看所有pcm的name
+	//print_all_pcm_name();
+	
+	playback_handle = open_sound_dev(SND_PCM_STREAM_PLAYBACK,PCM_NAME,RATE,FORMAT,CHANNELS,&period_frames);
+	if(!playback_handle) {
+		printf("cannot open for playback\n");
+        return -1;
+    }
+	
+	usleep(5);
+	
+	capture_handle = open_sound_dev(SND_PCM_STREAM_CAPTURE,PCM_NAME,RATE,FORMAT,CHANNELS,NULL);
+	if(!capture_handle) {
+		printf("cannot open for capuure\n");
+		snd_pcm_close(playback_handle);
+        return -1;
+    }
+	
+	if((err = snd_pcm_prepare(playback_handle)) < 0) {
+		printf("cannot prepare audio interface for use (%s)\n",
+			 snd_strerror(err));
+		goto out;
+	}
+
+	if((err = snd_pcm_prepare(capture_handle)) < 0) {
+		printf("cannot prepare audio interface for use (%s)\n",
+			 snd_strerror(err));
+		goto out;
+	}
+	
+	//打开要播放的PCM文件
+	pfd = open(PLAYBACK_FILE,O_RDONLY,0644);
+	if(pfd < 0){
+		printf("open %s error!!!\n",PLAYBACK_FILE);
+		goto out;
+	}
+	
+	//新建一个进程, 子进程播放, 父进程录音
+	p = fork();
+	if(p < 0) {
+		printf("fork error!!!\n");
+		goto out;
+	}
+	
+	if(p==0) {
+		char *pbuf;
+		int size,period_bytes;
+		period_bytes = snd_pcm_frames_to_bytes(playback_handle,period_frames);
+		pbuf = malloc(period_bytes);
+		while( size = read(pfd, pbuf, period_bytes)) {
+			//解决最后一个周期数据问题
+			if(size < period_bytes) {
+				memset(pbuf+size, 0, period_bytes-size);
+			}
+			
+			//size2frames = snd_pcm_bytes_to_frames(playback_handle,size);
+			size2frames = period_frames;
+			
+			//向PCM写入数据，播放
+			err = snd_pcm_writei(playback_handle, pbuf, size2frames);
+			if(err != size2frames) {
+				printf("write to audio interface failede err:%d (size2frames:%d)\n",err,size2frames);
+				free(pbuf);
+				close(pfd);
+				exit(-1);
+			}
+			printf("process:playback wrote %d frames\n",size2frames);
+			usleep(100);
+		}
+		
+		free(pbuf);
+		close(pfd);
+		sleep(1);	//等待一下，给点时间父进程录音
+		exit(0);
+	}
+	
+	char *cbuf;
+	const int frames_size = snd_pcm_frames_to_bytes(capture_handle,period_frames);
+	cbuf = malloc(frames_size);
+	memset(cbuf, 0, frames_size);
+	
+	//打开录音的保存文件
+	cfd = open(CAPTURE_FINE,O_RDWR | O_TRUNC | O_CREAT,0644);
+	if(cfd < 0){
+		printf("open %s error!!!\n",CAPTURE_FINE);
+		goto out;
+	}
+	
+	while(waitpid(p, NULL, WNOHANG) == 0) {	//查看一下子进程是否已经退出
+		//向PCM读一周期数据
+		if((size2frames = snd_pcm_readi(capture_handle, cbuf, period_frames)) < 0) {
+			printf("read from audio interface failed (%d)\n",size2frames);
+			free(cbuf);
+			close(cfd);
+			goto out;
+		}
+		printf("--process:capture read %d frames\n",size2frames);
+		write(cfd,cbuf,snd_pcm_frames_to_bytes(capture_handle,size2frames));
+		memset(cbuf,0,frames_size);
+		usleep(100);
+	}
+	free(cbuf);
+	close(cfd);
+
+	
+out:
+	snd_pcm_close(playback_handle);
+	snd_pcm_close(capture_handle);
+	printf("program finish ...\n");
+	return 0;
+}
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